Airbus Cross Bleed
In short thre intent of the cross bleed valve is to allow the interconnection of the two sides of the pneumatic manifold. This will now allow any of the operating air supplies to provide for the system(s) i.e pack, anti-ice (wing/engine) to continue to function that were supplied from the opposite side. This can as well be use to start the other engine(s) either on the ground or in the air.
With regard to your other question about the MEL. This is aircraft and operator specific as each aircraft has certain requirememts for this kind of opeartion. The other is dependant on the operator's approved operation,i.e. 180 or 240 Min. ETOPS.
Hope this is helpfull to you fornow.